The overall objective of this research is to purify tumor antigens of defined specificities from spent culture medium of melanoma cell lines and develop sensitive immunological tests, e.g., radioimmunoassay (RIA) and enzyme immunoassay (EIA), for detection and quantification of such antigens in the serum and other body fluids of cancer patients. Two antigens have been detected in the spent culture medium of a melanoma cell line that are immunogenic in the host. One antigen is of fetal origin and has been termed as fetal antigen (FA). FA is a glycoprotein of 60 to 70 kilodaltons. The other antigen is a lipoprotein of 180 to 190 kilodaltons, and has been observed to be preferentially expressed by melanoma cells and rarely by other tumor cells. Procedures have been developed to detect the presence of melanoma-associated antigen (MAA) circulation in human sera, both in free form and in association with immune complexes. The MAA has been further characterized physicochemically. It has a beta-electophoretic mobility. Upon delipidation of MAA with 2-chloroethanol and further purification by Sephadex LH 20 chromatography, it was observed that the protein portion is comprised of two polypeptides. These polypeptides are 17.5 and 26 kilodaltons as determined by SDS-PAGE under reducing condition. In addition, affinity procedures coupled with immunoprecipitation with allogeneic antibody have been developed to identify various antigenic components of immune complexes circulating in melanoma patients. In one instance, at least five different polypeptide chains were recognized. Of these, two subunits, 92 and 41 kilodaltons were predominant. These polypeptides are quite different, at least on the basis of molecular weight, from MAA. (2)